Silicone hydrogel (SiHy) contact lenses are widely used for correcting many different types of vision deficiencies. They are made of a hydrated, crosslinked polymeric material that contains silicone and a certain amount of water within the lens polymer matrix at equilibrium. According to the FDA's contact lens classification, hydrogel contact lenses are generally classified into two main categories: low water content contact lenses (containing less than 50% of water) and high water content contact lenses (containing greater than 50% of water). For SiHy contact lenses, high oxygen permeability, which is required for a contact lens to have minimal adverse effects upon corneal health, is achieved by incorporating silicone, not by increasing water content, in the crosslinked polymeric material. As a result, unlike conventional hydrogel contact lenses, SiHy contact lenses can have a low water content while still having a relatively high oxygen permeability (Dk), for example, Focus® Night & Day® from CIBA Vision Corporation (ca. 23.5% H2O and Dk˜140 Barrers; Air Optix® from CIBA Vision Corporation (ca. 33% H2O and Dk˜110 Barrers); PureVision® from Bausch & Lomb (ca. 36% H2O and Dk˜100 Barrers); Acuvue® Oasys® from Johnson & Johnson (ca. 38% H2O, Dk˜105 Barrers); Acuvue® Advance® from Johnson & Johnson (ca. 47% H2O, Dk˜65 Barrers); Acuvue® TruEye™ from Johnson & Johnson (ca. 46% H2O, Dk˜100 Barrers); Biofinity® from CooperVision (ca. 48% H2O, Dk˜128 Barrers); Avaira™ from CooperVision (ca. 46% H2O, Dk˜100 Barrers); and PremiO™ from Menicon (ca. 40% H2O, Dk˜129 Barrers).
Water in a SiHy contact lens can provide the desirable softness that enable a SiHy lens to be worn for sufficiently long periods of time and provides patients with the benefits including adequate initial comfort (i.e., immediately after lens insertion), relatively short period of adapting time required for a patient to become accustomed to them, and/or proper fit. Higher water content would be desirable for providing SiHy contact lenses with biocompatibility and comfort. But, there is a limit to the amount of water (believed to be 80%) that a SiHy contact lens can contain while still possessing sufficient mechanical strength and rigidity required for a contact lens, like conventional hydrogel contact lenses. Moreover, high water content could also have undesired consequences. For instance, oxygen permeability of a SiHy contact lens could be compromised by increasing water content. Further, high water content in a SiHy lens could result in greater in-eye dehydration and consequently dehydration-induced wearing discomfort, because a SiHy contact lens with a high water content could deplete the limited supply of tears (water) of the eye. It is believed that in-eye dehydration may be derived from evaporation (i.e., water loss) at the anterior surface of the contact lens and such water loss is primarily controlled by water diffusion through a lens from the posterior surface to the anterior surface, and that the rate of diffusion is closely proportional to the water content of the lens bulk material at equilibrium (L. Jones et al., Contact Lens & Anterior Eye 25 (2002) 147-156, herein incorporated by reference in its entirety).
Incorporation of silicone in a contact lens material also has undesirable effects on the biocompatibility of the contact lens, because silicone is hydrophobic and has great tendency to migrate onto the lens surface being exposed to air. As a result, a SiHy contact lens will generally require a surface modification process to eliminate or minimize the exposure of silicone of the contact lens and to maintain a hydrophilic surface, including, for example, various plasma treatments (e.g., Focus® Night & Day® and Air Optix® from CIBA Vision Corporation; PureVision® from Bausch & Lomb; and PremiO™ from Menicon); internal wetting agents physically and/or chemically embedded in the SiHy polymer matrix (e.g., Acuvue® Oasys®, Acuvue® Advance® and Acuvue® TruEye™ from Johnson & Johnson; Biofinity® and Avaira™ from CooperVision). Although surface modification techniques used in the commercial SiHy lens production may provide fresh (unused) SiHy lenses with adequately hydrophilic surfaces, a SiHy lenses worn in the eye may have dry spots and/or hydrophobic surface areas created due to air exposure, shearing forces of the eyelids, silicone migration, and/or partial failure to prevent silicone from exposure. Those dry spots and/or hydrophobic surface areas are non-wettable and susceptible to adsorbing lipids or proteins from the ocular environment and may adhere to the eye, causing patient discomfort.
Therefore, there are still needs for SiHy contact lenses with hydrophilic surfaces that have a persistent hydrophilicity, wettability, and lubricity that can be maintained in the eye throughout the entire day.